Raman amplifiers in optical communication systems operate through co- and/or counter-propagating pump wavelengths on an optical fiber with data-bearing channels. For C-band (e.g., 1530-1565 nm), the pump wavelengths are in the 1400 nm range (e.g., about 90 nm below the data-bearing channels). As capacity continues to increase and network operators are either fiber count constrained or have a high cost for adding additional fiber pairs, there is a drive to migrate to C and L dual-band systems (C+L) to increase the capacity of the link once the C-band is filled (the L-Band is 1565 nm-1625 nm). A C+L dual-band system requires a wider band distributed Raman amplifier to amplify the C and L band in the fiber. The C+L band Raman amplifier would require multiple Raman pump wavelengths (e.g., five to six) to support the wider C and L band systems. The design choice would be determined by factors such as Raman amplifier ripple, maximum available pump power, cost, etc. The pumps are distributed between 1424 nm and 1500 nm. If the fiber Zero Dispersion Wavelength (ZDW) of the transmission fiber falls in a certain region, there will be efficient Four-Wave Mixing (FWM) between the pumps because of good phase matching between pairs of pumps. The FWM can be degenerate four mixing or non-degenerate four-wave mixing.
One approach to suppress FWM between Raman pump wavelengths is to modulate the pumps out of phase. However, modulating the pumps increases the requirement of peak pump power and also increases the peak pump power in fiber, which increases the penalty from double Rayleigh backscattering at the signal wavelength.